我尝试使用画布元素在 Javascript 中为跟随光标的球设置动画。我将对画布对象的引用传递给名为 followMouse 的函数，但当我尝试获取其上下文时，firebug 给出了此错误：

TypeError: canvas.getContext is not a function

当我将画布引用记录到控制台时，它会显示画布元素。我不知道为什么我无法在函数中获取上下文。有人知道发生了什么吗？

这是我的代码：

function drawCircle(x, y, canvas) {
    var context = canvas.getContext('2d');
    context.beginPath();
    context.arc(x, y, 40, 0, 2 * Math.PI);
    context.fill(); 
}

function followMouse(canvas, mousePos) {
    var context = canvas.getContext("2d");
    console.log(canvas);
    context.clearRect(0, 0, 700, 700);
    var xPos = xPos + (mousePos.x - xPos) / 100;
    var yPos = yPos + (mousePos.y - yPos) / 100;
    drawCircle(xPos, yPos, canvas);
    window.requestAnimFrame(followMouse, canvas);
}

window.onload = function() {
    var canvas = document.getElementById("main");

    var mousePos = {
        x: 0,
        y: 0
    };

    canvas.addEventListener('mousemove', function(evt) {
           var pos = getMousePos(canvas, evt);
           mousePos.x = pos.x;
           mousePos.y = pos.y;
    }, false);

    followMouse(canvas, mousePos);
}